The invention relates to optical amplifiers, and more particularly to optical amplifiers designed to be used in optical fiber transmission systems with wavelength division multiplexing (WDM). Such amplifiers are used at regular intervals to compensate line losses. The amplifiers over an entire link are preferably identical, presenting gain that is as flat as possible over the entire wavelength band used in the transmission system, for given input power. One of the problems encountered when installing optical fiber transmission systems is that the line losses over the link sections between the various amplifiers present values that are different; under such circumstances, the amplifiers receive different powers, such that system performance can be improved if each amplifier is matched to the line loss of the section of fiber that precedes it, thereby ensuring gain that is flat in spite of variations in applied input power.
To perform such matching, proposals have been made to give each amplifier gain that is greater than the gain theoretically required, and to associate each amplifier with a variable optical attenuator enabling the gain of each assembly comprising an amplifier plus an attenuator to be reduced to the value that is required, as a function of line losses in the section concerned. In that solution, the loss of each optical attenuator is adapted so that the sum of line loss over the section concerned plus the loss in the attenuator is constant. That solution degrades the performance of the link in terms of noise: the gain value selected for each amplifier is the value that corresponds to the maximum line loss possible over the section concerned.
Another solution for matching amplifiers consists in deliberately varying the length of doped optical fiber to match the requirements of the various amplifiers. That solution is complex to implement and it is expensive.
An object of the invention is thus to provide a solution enabling the gain of an optical amplifier to be adapted easily, e.g. as function of line losses over the section of optical fiber that precedes the amplifier in a transmission system, so as to ensure flat gain for the amplifier. Another object of the invention is to minimize losses and maximize signal-to-noise ratio as a result of such matching.
An article by Kyo Inoue et al., entitled "Tunable gain equalization using a Mach Zender optical fiber in multistage fiber amplifiers" published in IEEE Photonics Technology Letters, Vol. 3, No. 8 (1991), proposes using a Mach Zender optical filter to adapt the gain of a three-stage erbium-doped fiber amplifier (EDFA) in a wavelength division multiplex transmission system having 29 channels. In that article, the problem encountered is that of variations in gain or "gain unbalance" in each of the amplifiers over the band of the multiplex. That defect can be accepted in each amplifier, but after several stages of gain, such gain unbalance becomes unaccepted over the band of the multiplex. To mitigate that problem, proposals are made to use the transmittance slope of a Mach Zender filter to compensate variations in gain induced over the band of the multiplex by three EDFA stages.
That article proposes a solution which differs from that of the invention and which applies to a different problem. The problem of that invention is to compensate for accumulated gain unbalance over the bandwidth of the filter, i.e. to equalize gain over the WDM to compensate for the accumulated gain unbalance on the various channels of the WDM.
The solution proposed reflects the problem. Firstly, the proposed circuit has only one filter for three amplification stages: after all the object is to correct equalization as rarely as possible, and only when that turns out be necessary, i.e. when accumulated gain unbalance is large; the article specifies that prior techniques are unsuitable for "adjustable compensation of accumulated gain unbalance". From this point of view, the teaching of that document goes against the solution of the present invention.
Finally, the solution proposed does not enable the amplifier to be matched to the power it receives, but serves at best to equalize accumulated gain in a single direction. This comes from the tuning range of the Mach Zender filter used; the filter cannot be tuned over a wavelength band that is sufficient to change the sign of the slope of the filter; in addition, even if that were possible, changing the sign of the slope of the filter would increase gain unbalance over the wavelength band of the multiplex. From this point of view also, the teaching of that document goes against the solution of the present invention.
Photonics Technologies of Pagewood, New South Wales, Australia, proposes under the trademark AmpFlat a gain-flattening filter. The purpose of that filter is to compensate gain unbalance over the band of an EDFA; it is designed to be added to most EDFAs for the purpose of improving performance. The filter is available in two forms: a "laboratory" form in which it is possible to adjust filter parameters, e.g. center frequency and extinction; and a "fixed" form in which the parameters are fixed. It is stated that the fixed form is available "when the response required of the filter is well defined" in "an environmentally-stable miniaturized version". That filter seeks to solve the same problem as does the above-mentioned article by Kyo Inoue, and its teaching goes against adjusting a filter on site. In any event, as in the above-specified article, it suggests using the filter to compensate for gain unbalance over the band of the multiplex, but not for matching the amplifier as a function of the powers it receives.